2We have discovered first homologous series of iron pnictide oxide superconductors (Fe 2 As 2 )(Ca n+1 (Sc,Ti) n O y ) [n = 3,4,5]. These compounds have extremely thick blocking layers up to quintuple perovskite oxide layers sandwiched by the Fe 2 As 2 layers. These samples exhibited bulk superconductivity with relatively high T c up to 42 K. The relationship between T c and the iron-plane interlayer distance suggested that superconductivity due to the mono Fe 2 As 2 layer is substantially 40 K-class.In a recently discovered layered iron pnictide family 1 , the superconducting transition temperature T c increased under ambient pressure in the order of FeSe (T c = 8-13 K, d = 5.49 Å) 2, (Ba,K)Fe 2 As 2 (38 K, 6.65 Å) 3 and SmFeAs(O,F) (55 K, 8.44 Å) 4 , where d represents the distance between the iron-planes. This suggested that emphasis of two-dimensionality by an increase of the interlayer distance of superconducting planes would be a promising method to develop new high-T c superconductors. On the other hand, a new family of layered iron pnictides with perovskite-type oxide blocking layers has recently been discovered 5-12 . The thickness of the perovskite-type layer can be controlled according to the composition and the synthesis conditions; therefore, an attempt to develop new layered iron pnictides with thicker oxide layers may result in a new series of homologous superconductors. Novel compounds with double or triple perovskite layers were discovered in our recent study, such as (Fe 2 As 2 )(Ba 3 Sc 2 O 5 ) and (Fe 2 As 2 )(Sr 4 (Sc,Ti) 3 O 8 ) 12 . These new compounds indicated that a perovskite layer, which can maintain electroneutrality with the (Fe 2 As 2 ) -2 layer and have an appropriate lattice size a~4 Å, can form layered iron pnictide oxides. According to these 3 empirical observations, three new compounds with different crystal structures, (Fe 2 As 2 )(Ca n+1 (Sc,Ti) n O y ) [n = 3,4,5 and y ~3n-1] were discovered.Samples were synthesized by solid-state reaction from starting materials of FeAs (3N), Ca (2N), CaO (2N), Ti (3N), TiO 2 (3N) and Sc 2 O 3 (3N). The starting reagents were moisture sensitive, so that manipulation was carried out in a glove box filled with argon gas. Powder mixtures were pelletized, sealed in evacuated quartz ampoules and heated at 1000-1200 °C for 60-100 h followed by slow cooling to room temperature.Phase identification was carried out using powder XRD (Rigaku Ultima-IV) and intensity data were collected in the 2θ range of 3-80° at a step of 0.02° using Cu-K α radiation. High resolution TEM images were taken using JEM-2010F and JEM-4010 (JEOL). Magnetic susceptibility was measured using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-XL5s). Electrical resistivity was evaluated by the AC four-point-probe method (Quantum Design PPMS). (Fe 2 As 2 )(Ca 4 (Sc 0.67 Ti 0.33 ) 3 O y ), (Fe 2 As 2 )(Ca 5 (Sc 0.5 Ti 0.5 ) 4 O y ) and (Fe 2 As 2 )(Ca 6 (Sc 0.4 Ti 0.6 ) 5 O y ) samples were successfully obtained by sintering for 72 h at 1050,...

Influence of the upper critical-field anisotropy on the transport properties of polycrystalline Mg B 2

Many superconductors do not entirely expel magnetic flux-rather, magnetic flux can penetrate the superconducting state in the form of vortices. Moving vortices create resistance, so they must be 'pinned' to permit dissipationless current flow. This is a particularly important issue for the high-transition-temperature superconductors, in which the vortices move very easily. Irradiation of superconducting samples by heavy ions produces columnar defects, which are considered to be the optimal pinning traps when the orientation of the column coincides with that of the vortex line. Although columnar defect pinning has been investigated using macroscopic techniques, it has hitherto been impossible to resolve individual vortices intersecting with individual defects. Here we achieve the resolution required to image vortex lines and columnar defects in Bi2Sr2CaCu2O8+delta (Bi-2212) thin films, using a 1-MV field-emission electron microscope. For our thin films, we find that the vortex lines at higher temperatures are trapped and oriented along tilted columnar defects, irrespective of the orientation of the applied magnetic field. At lower temperatures, however, vortex penetration always takes place perpendicular to the film plane, suggesting that intrinsic 'background' pinning in the material now dominates.

Superconductors can be used as dissipation-free electrical conductors as long as vortices are pinned. Vortices in high-temperature superconductors, however, behave anomalously, reflecting the anisotropic layered structure, and can move readily, thus preventing their practical use. Specifically, in a magnetic field tilted toward the layer plane, a special vortex arrangement (chain-lattice state) is formed. Real-time observation of vortices using high-resolution Lorentz microscopy revealed that the images of chain vortices begin to disappear at a much lower temperature, Td, than the superconducting transition temperature, Tc. We attribute this image disappearance to the longitudinal oscillation of vortices along the chains.

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